These aren't isolated incidents — there was another human plague case in Sante Fe county this year and four cases each in New Mexico in 2016 and 2015. There are usually a few plague infections in the US every year, mostly in the West.

References to "the plague" call to mind medieval times, with masked plague doctors wandering the streets and a "Black Death" spread by rats (or really, the fleas they carry).

Plague cases now aren't too far from that. As the CDC explains, it's still a potentially fatal and serious bacterial illness. Different forms of the plague manifest in different ways. Flea bites spread bubonic or septicemic plague. Both cause fever and weakness. Bubonic plague results in painfully swollen lymph nodes; septicemic plague happens when the infection gets in the blood and causes skin and tissue to turn black and die. It can appear on its own or develop from bubonic plague.

Untreated, patients can develop pneumonic plague, the most serious form of the disease, which spreads when infected people cough droplets into the air.

The Yersinia pestis bacteria that circulate and cause plague today are remarkably similar to the bacteria researchers have identified as the cause of the Black Death. It's just that today, we live in an era of antibiotics — as long as the disease is caught early, we can treat it.

Why plague persists

It's possible that people can catch plague from direct contact with infected animals, according to the New Mexico Department of Health, but fleas are still usually the link between infected rodents and humans.

"Pets that are allowed to roam and hunt can bring infected fleas from dead rodents back into the home, putting you and your children at risk," said Paul Ettestad, public health veterinarian for the Department of Health, in a news release.

In wild rodent populations that harbor the bacteria, plague can thrive for a long time before humans come into contact with it. In the American West, where most US cases occur, there are a number of rodents— from rats to voles to prairie dogs — that are susceptible to plague, Ken Gage, a researcher focused on vector-borne disease at the Centers for Disease Control and Prevention, told NPR in 2014. In rural areas with semi-arid forests and grasslands, these animals spread the disease amongst themselves.

"What we see in the West here is the fleas will crawl up to the entrance of the burrow and wait for a host to come by," Gage told NPR. "If they get on another rodent that they can live on, then they've been successful. But they can also jump on humans, or on dogs or coyotes or cats, which aren't the right hosts, but unfortunately those animals can be bitten by the fleas and get plague."

Still, the US only gets a handful of cases — usually between one and 17 every year. Plague is a bigger problem in places that have a harder time shutting down outbreaks. In the US, disease detectives try to find every person an infected individual came into contact with. That's harder in regions with humanitarian crises or ongoing conflicts, according to the World Health Organization.

In the 1400s, approximately 50 million people died of the Black Death. It's still one of the scarier infectious diseases out there, with a mortality rate between 30% and 60% if untreated. But fortunately, antibiotics make it possible to treat most cases now. Between 2010 and 2015, there were around 584 deaths.